COTTON YIELD AND QUALITY By Hal Lewis For presentation to 16 th annual Engineered Fiber Selection Conference, Greenville, SC, June 10, 2003. INTRODUCTION Yield Trends: US cotton yields increased dramatically from the early 1970 s up to the mid 1980 s. From about 1985 through 2000 yields declined steadily reaching a low in the late 1990 s early 2000 s. This yield decline is best characterized by a high level of instability, with year to year variance nearly quadrupling in recent years as compared to the 1970 s. Quality Trends: In 1930, approximately 75 percent of the cotton acreage in the U.S. was planted with short staple varieties, measuring 1 inch or less. By 1950, the short staple varieties accounted for only about one third of the total production. Staple was improved by about 0.05 of an inch over the 25 year period, 1974 1999. Between 1979 and 1984, the staple increased from about 1.05 inches to about 1.08 inches, and between 1990 and 1991 from about 1.08 to about 1.10 inches. The most noteworthy aspect of this situation is that the staple has been more or less static since 1991, with the exception of a precipitous drop to a near 1983 era level of 1.07 and 1.06 inches in 1998 and 1999, respectively. From 1975 through the early 1990 s the average micronaire value of the US crop was good and did not vary widely. Beginning in 1993 the average micronaire value moved up abruptly to about 4.35 and then steadily increased to a high of approximately 4.66 in 2001. This has become a serious problem. Fiber strength faltered in the mid 1990 s and lost significant ground. Much of the decline in fiber strength (grams/tex) may be related to the dramatic increase in micronaire value. Magnitude vs. Uniformity: Improvement in fiber quality has been focused on improvements in magnitude of fiber properties. Is it time for US cotton to refocus on the uniformity of cotton fibers, or reduction in the level of variance among fibers in the bale? Is not SJV cotton better than Memphis/Southeastern because it is more uniform in quality, not because it is always longer and stronger? Sources Of Variance: American upland cotton is a highly indeterminate, perennial plant which is grown in an annual cropping system. Fruiting forms are produced on cotton plants in a sequential manner, starting with the first fruiting branch at the bottom of the plant and progressing upward in a highly predictable manner. The developmental sequence is commonly defined as about three days difference in age for first position bolls from one fruiting branch to the next, and approximately six days difference in age for fruiting forms produced in second positions and further out the same fruiting branch. This indeterminate fruiting habit provides for significant sources of variation in cotton fibers simply because
2 the fiber from bolls produced in different positions on the plant may have different fiber properties because they were produced and developed at different times under quite different conditions of temperature, moisture etc. Thus, the developmental genetic mechanisms of the cotton plant provide an opportunity for variation in fiber properties. However, the most pernicious source of variability is the environmental conditions under which the fibers develop. The data presented above clearly show that there is a great deal of variation in cardinal fiber properties up and down and out the fruiting branches on cotton plants, which result in similar variation among the fibers in a bale of cotton. The important question is, what can be done to improve this situation? INCENTIVES FOR QUALITY IMPROVEMENT Avoid Discounts. Examination of the Official USDA loan schedule for fiber quality premiums and discounts reveals only 16 minor premiums and about 90 discounts, or a ratio of nearly 6 discounts for 1 premium. Thus, the primary motivation for producers to strive for improved fiber quality is to improve yield and avoid discounts. For example, producers lost about 100 million dollars to high micronaire discounts alone from the 2001 crop. This represents an average loss of about 6 dollars per each bale produced in 2001. Approximately 25 million dollars of this loss was from the mid south crop. Producers have no choice but to pay attention to fiber quality. Improve Yield: It may well be that the most important fiber property is yield. Thus, it is very important to understand how yield components and fiber quality parameters interact in order to produce profitable yields with desirable fiber quality. The prime question may be, is managing the crop for enhanced fiber quality consistent with managing for better yield? Yield Components Cotton lint yield is probably best understood in terms of the components, which make it up. Fiber or lint yield in cotton is determined by two (2) major components, i.e., the number of seeds produced per acre and the weight of fiber produced on the seed. Cotton fibers are elongated epidermal cells of the outer integument of the seed coat. No seed - no fiber. Lint Yield = [(No. of Seeds/Acre)(Weight of Fiber /Seed)] The structure and dimensions of cotton fibers determine their quality. Fiber dimensions are length and micronaire. Fiber structure is primarily involved with strength and is specified by the molecular components and the arrangement of these molecules in fiber cell walls. Official strength values in current commercial use are directly influenced by micronaire. Grams per tex is derived from the intrinsic fiber strength in grams force to break divided by the micronaire value expressed in tex units. Tex is the metric equivalent of micronaire. Officially,
3 micronaire has no units of measurement but is understood to be highly correlated with micrograms per inch of fiber length. The units of tex are micrograms per millimeter of fiber length. Therefore, if the intrinsic fiber strength remains constant and the micronaire value increases, then tenacity, or grams per tex, will decrease. The higher the micronaire value, at a constant intrinsic strength, the lower the tenacity or grams per tex. Strength and length are highly correlated. MICRONAIRE AND YIELD There has been much discussion about high micronaire values in recent years. Essentially, mills complain about difficulties encountered in spinning higher micronaire cottons and growers are devastated by high micronaire discounts, as mentioned above. The important question from the producer s point of view is can we lower micronaire without giving up yield potential? Some individuals take the position that genetically lowered micronaire mandates reduced yield potential. While there is a theoretical relationship between micronaire and yield potential, in practice there has never been an actual relationship between the micronaire of the U.S. crop and yield. In fact, between 1975 and 2001 the correlation between micronaire and yield has been practically non-existent. Regression analysis of micronaire versus yield gives a coefficient of determination of about 0.03, which indicates no relationship has existed over this 27-year period. This status of affairs indicates that growers should manage their crops to obtain acceptable yields with non-discount micronaire levels. This can be approached by planting varieties with lower micronaire levels and high yield potential. If varieties that meet these criteria cannot be found, then, growers can manage their crops to minimize micronaire discounts. Technology is available to do this. This technology is based on the knowledge that bolls in the core of the plant produce the highest micronaire lint, and bolls on the periphery of the plant produce the lowest micronaire lint. The procedure involves finding out what the core micronaire level is and then timing harvest aid applications to obtain a blend of the high micronaire lint with the low micronaire lint so as to avoid discounts and optimize crop value. STAPLE AND YIELD The relationship between fiber length and yield is seldom mentioned, however, regression analysis of fiber length versus yield of the U.S. crop from 1975 through 2001 reveals a very strong correlation. This suggests that growers would be well advised to manage their crops very carefully to insure the longest staple possible. The first approach should be to choose varieties with good yield potential and good staple. Once this is done, the crop must be managed to enhance fiber initiation and development with regard to the weight of fibers per seed and fiber length. The number of fibers per seed is highly correlated to the
4 weight of fiber per seed. Fiber initiation begins about a day before flowering and continues for about 4 5 days. Only a minor stress at this time can cause a significant problem with fiber initiation and the weight of fiber per seed. Once fibers are initiated, they elongate rapidly, attaining approximately 90% of their mature length in the first 2 weeks. Nevertheless, fiber elongation continues up to about 35 days after flowering. Many of the varieties we are growing today elongate up to 0.2 of an inch after 20 days past flowering. This means lint may increase in staple by as much as 6/32nds of an inch after 20 days past flowering. Fibers initiate rapidly and elongate rapidly. This indicates that stress should, resolutely, be minimized in the fiber initiation and early elongation stages if good staple and, perhaps, yield are to be obtained. The most pernicious stress usually encountered at this stage is drought. In the delta region, cotton plants normally produce between 12 and 18 effective fruiting branches. Up to 75% of total yield may be produced by first position bolls on the first 8 to 10 fruiting branches. This indicates that moisture availability should be maintained at non-stress levels for about 40-50 days after flowering. It is, of course, important that moisture stress should be avoided in the week immediately prior to the onset of flowering. Variation in lint yield is highly correlated with the number of fibers per seed and less well correlated with the weight per fiber. WHITE SPECK The white speck has become an item of much discussion recently. Reportedly this problem is associated with dead fiber or fibers which are extremely immature. Existing data indicate these fibers have a micronaire value of 2,0 or less, which is the reported micronaire value of the primary cell wall; UHM length of 1.0 inch or less; and strength of 20 gm/tex or less. The origin of these fibers appears to be late bolls on the periphery of the cotton plant which fail to mature but open and fluff sufficiently to be, at least, partially extracted by a spindle type cotton picker. This problem may have been exacerbated by the large scale adoption of transgenic varieties. Two factors appear to be involved: first, the increased cost of planting seed has encouraged farmers to plant fewer seeds per foot of row, resulting in thinner stands or lower plant populations. Such management practices result in a larger number of later maturing bolls being produced on the periphery of the plants. In addition, many of the transgenic varieties appear to be later in maturity, resulting in more late season bolls. Both of these tendencies may result in an increased probability for dead fibers in the bale. This is a solvable problem, given the proper incentives and motivation.
5 What Went Wrong? At the Midsouth GinShow earlier this year, the Grand Vizier of cotton merchants raised the question What went wrong? The question related to the quality of the 2002/03 Memphis/Southeastern cotton crop. Actually, the intent of the question probably could have been more accurately stated as; Who did this to us? The merchant s major concern seemed to be that he could not sell this cotton into the competitive market at a satisfactory profit. I have no doubt that there is some truth in this perspective, however, a more appropriate question would have been What did we do wrong? The truth is that producers have responded properly to the message from the market place, that is, quality has little or no monetary value. The opportunities available to producers for enhanced profitability are primarily, increased yields, reduced production costs and avoidance of discounts. There is little incentive to improve quality above the base levels of the loan chart. This means that the answer to the question, what did we do wrong, is, we failed to provide market incentives for the production of the cotton qualities desired by the market. Thus, the error, or wrong, was perpetrated not by the production side of the industry but, instead, by the utilization side. The implied accusation in these questions is that cotton producers must be blamed for these problems. The truth is that producers have had little to say about the policies which led to this problem. Now we come to the crux of the questions, i.e, how do we put it right? We do owe Mr. Dunavant for using his prestige to bring attention to the problem. Now, we must ask him and the leaders of the other segments of the industry to address the solution, or the answer to the last question. There must be a mechanism whereby the value of fiber quality can be properly identified by the market! Make no mistake about it, once the market places adequate monetary value on the kinds of fiber quality it wants, the production side of the industry will respond vigorously.